1. Field of the Invention
The present invention relates to a display device having a sealing structure. Specifically, the invention relates to an active matrix display device using a semiconductor element (an element formed of a semiconductor thin film) and to electronic equipment employing the active matrix display device as its display unit.
2. Description of the Related Art
An electro-optical device such as an EL display device having an EL element has lately been attracting attention as a flat display.
The EL element has a structure in which an EL layer is sandwiched between a pair of electrodes (an anode and a cathode). The EL layer generally takes a laminate structure. A typical example of the laminate structure is the one proposed by Tang et al. of Eastman Kodak Company, and consists of a hole transportation layer, a light emitting layer and an electron transportation layer. This structure has so high a light emission efficiency that it is employed in almost all of EL display devices that are under development at present.
Other examples of the EL layer laminate structure include a structure consisting of a hole injection layer, a hole transportation layer, a light emitting layer, and an electron transportation layer which are layered in this order on an anode, and a structure consisting of a hole injection layer, a hole transportation layer, a light emitting layer, an electron transportation layer, and an electron injection layer which are layered in this order on an anode. The light emitting layer may be doped with a fluorescent pigment or the like.
In this specification, all of the layers provided between a cathode and an anode are collectively called an EL layer. Accordingly, the hole injection layer, a hole transportation layer, a light emitting layer, an electron transportation layer, an electron injection layer, etc. mentioned above are all included in the EL layer.
A given voltage is applied to the EL layer structured as above from the pair of electrodes, whereby recombination of carriers takes place in the light emitting layer to emit light. An EL element emitting light is referred to herein as EL element being driven. The EL element in this specification refers to a light emitting element composed of an anode, an EL layer, and a cathode.
The EL element in this specification refers to both a light emitting element that utilizes light emission from a singlet exciton (fluorescence) and a light emitting element that utilizes light emission from a triplet exciton (phosphorescence).
An active matrix structure is given as one of structures for EL display devices.
FIG. 9 shows an example of the structure of a pixel portion in an active matrix EL display device. In the active matrix EL display device, each pixel has thin film transistors (hereinafter referred to as TFTs). Each pixel has a switching TFT 901 whose gate electrode is connected to one of gate signal lines (G1 to Gy) for inputting a selection signal from a gate signal line driving circuit. The switching TFT 901 in each pixel has a source region and a drain region one of which is connected to one of source signal lines (S1 to Sx) for inputting a signal from a source signal line driving circuit and the other of which is connected to a gate electrode of an EL driving TFT 902 and to one of electrodes of a capacitor 903 that is provided in each pixel. The other electrode of the capacitor 903 is connected to one of power supply lines (V1 to Vx). The EL driving TFT 902 provided in each pixel has a source region and a drain region one of which is connected to one of the power supply lines (V1 to Vx) and the other of which is connected to an EL element 904 that is provided in each pixel.
When the gate signal line driving circuit selects the gate signal line GI and inputs a signal, the switching TFT 901 that is connected to the gate signal line G1 is turned ON. If the source signal line driving circuit inputs a signal to the source signal lines Si to Sx at this point, the EL driving TFT 902 is turned ON in every pixel to which the signal is inputted. Thus a current flows into the EL element 904 from its associated power supply line (one of V1 to Vx), causing the EL element 904 to emit light. This operation is repeated for all of the gate signal lines G1 to Gy to display an image.
The EL element 904 has an anode, a cathode, and an EL layer that is provided between the anode and the cathode. If the anode of the EL element 904 is connected to the source region or the drain region of the EL driving TFT 902, the anode of the EL element 904 serves as a pixel electrode whereas the cathode thereof serves as an opposite electrode. On the other hand, if the cathode of the EL element 904 is connected to the source region or the drain region of the EL driving TFT 902, the cathode of the EL element 904 serves as the pixel electrode whereas the anode thereof serves as the opposite electrode.
The electric potential of the opposite electrode is called herein as an opposite electric potential. A power supply for giving the opposite electric potential to the opposite electrode is called an opposite power supply. The electric potential difference between the electric potential of the pixel electrode and the electric potential of the opposite electrode corresponds to an EL driving voltage, which is applied to the EL layer.
An organic EL layer has a problem of being degraded by moisture or oxygen. For that reason, it is common to seal the device after the EL layer is formed by a UV-curable resin in a nitrogen atmosphere instead of exposing the device to the air. FIGS. 4A and 4B show an example of sealing the EL display device.
FIG. 4A is a top view of the EL display device. A pixel portion 402 having EL elements, a gate signal line driving circuit 403, and a source signal line driving circuit 404 are formed on an insulating substrate 41. A sealing member 401 is formed on the insulating substrate 41 so as to surround the pixel portion 402, the gate signal line driving circuit 403 and the source signal line driving circuit 404. At this point, an opening (not shown) is formed as an inlet for injecting a filer 43 later. Then a spacer (not shown) is sprayed to bond a covering member 42. After the sealing member 401 is cured by irradiation of ultraviolet rays, the filler 43 is filled in a region enclosed with the covering member 42 and the sealing member 401. The inlet for the filler 43 is then sealed by an end-sealing material (not shown).
FIG. 4B is a sectional view taken along the line A–A′ in FIG. 4A.
In order to simplify the illustration, components shown in FIG. 4B are limited to a TFT 413 constituting the gate signal line driving circuit 403, and an EL driving TFT 414 and an EL element 417 which constitute the pixel portion 402. The insulating substrate 41 is referred to as pixel substrate. The EL element 417 is composed of a pixel electrode 407, an EL layer 416, and an opposite electrode 408. The covering member 42 is set in place by the sealing member 401. The filler 43 is sealed in a space between the pixel substrate 41 and the covering member 42. A hygroscopic substance (not shown) is added to the filler 43. In this way, degradation of the EL element 417 due to moisture is avoided.
Denoted by 406 is a gate insulating film for the TFT 413 and for the EL driving TFT 414, and 415 denotes an interlayer insulating film.
A signal to be inputted to the pixel portion 402, the gate signal line driving circuit 403, and the source signal line driving circuit 404 is inputted through wiring lines 412 (412a to 412c) from an FPC (flexible printed circuit) substrate 410 (see FIG. 4A). The wiring lines 412 run between the pixel substrate 41 and the sealing member 401 to connect the FPC substrate 410 with the driving circuits. The FPC substrate 410 is connected at an external input terminal 409 to the wiring lines 412 through an anisotropic conductive film (not shown).
Any EL display device has to seal its EL element by bonding a pixel substrate to a covering member using a sealing member in order to prevent degradation of the EL element.
Unlike the pixel portion, portions where the sealing member and the driving circuits are formed do not display an image. In conventional display devices, these portions that are not used to display an image occupy a great proportion to hinder the display devices from reducing their sizes.